The present invention relates to heat transfer in a water heater in general, and to a water heater flue having heat exchange enhancing features on the inside of the flue in particular.
Heat exchange between a liquid and a gas is a process which has many industrial and domestic applications. Perhaps the most widely used domestic application of heat exchangers is domestic hot water which involves the transferring of heat from combustion gases to the water contained within a hot water heater. Typically a hot water heater has a tank which holds the water to be heated and a burner. The water is heated by the combustion of fuel with air in a burner to produce exhaust products which heat the bottom of the tank and are vented through a centrally located flue/heat exchanger which extends through the hot water tank. Two considerations which are paramount in the design of a hot water heater are durability and efficiency. Ever since the early 1970s there has been a heightened awareness of the importance of efficiency for cost, environmental, and geopolitical reasons. Efficiency is a measure of how effectively the heat energy present in the fuel is transferred to the water contained within the hot water heater tank.
Fuel is combusted with air to form hot gases that pass up through the central flue, exchanging heat with the wall of the flue and with the water contained within the water tank. It has long been known that internal baffles within the central flue can increase heat transfer between the flue gases and the water within the water tank. The baffles perform three basic functions: First, the baffles slow the passage of the combustion gases through the flue giving more time for heat transfer between the gases and the flue wall. Second, the baffles mix the combustion gases within the flue, bringing more of the flue gases into contact with the flue wall which transfers heat to the water. Third, the baffles conduct heat to the wall of the flue.
Water heaters with baffles in the flue have proven to have good heat transfer and durability. However, even small improvements in overall efficiency are desirable.